Omnidirectional beacon antenna having dipole radiator and parasitically fed horn radiator



3,141,169 IATOR July 14, 1964 H. M. BELLIS ETAL OMNIDIRECTIONAL BEACON ANTENNA HAVING DIPOLE RAD AND PARASITICALLY FED HORN RADIATOR 5 Sheets-Sheet 1 Filed Nov. 21. 1960 a Y. c 'm M w W E EM m T. h e C F H 2n r e 0 m Q r F Q0 a w 6 m. p. v a 0 he c A n A Hm. w W 9 N Q e a 4. s W 1 O L 'IIIIIII/IIIIIIIIII/Il/ll III/1 l I 7111/, 'IIIIIIIIIIIIIIIIIIII/llll m 3. ma W7. 0

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ATTORNE July 14, 1964 H. M. BELLIS ETAL 3,141,169

OMNIDIRECTIONAL BEACON ANTENNA HAVING DIPOLE RADIATOR ALLY FED HORN RADIATOR AND PARASITIC 5 Sheets-Sheet 2 Filed Nov. 21. 1960 .llllllflllfl'l/lll lilllllllldllllf 'llll/lllll.

H0 WA R0 M. 854 1. IS ER WIN J- HUBER FRANK E, kOL/NSRV ATTOKN Y ly 1964 H. M. BELLIS ETAL 3,141,169

OMNIDIRECTIONAL BEACON ANTENNA HAVING DIPOLE RADIATOR AND PARASITICALLY FED HORN RADIATOR 5 Sheets-Sheet 3 Filed Nov. 21. 1960 53 mmvroks.

HOWARD M, BELL/5 O ERW/N \l. HUB Q Q19, 7 a? FRANK e. KOLINSKY ATTORNEY July 14, 1964 H. M. BELLIS ETAL 3,141,169

OMNIDIRECTIONAL BEACON ANTENNA HAVING DIPOLE RADIATOR AND PARASITICALLY FED HORN RADIATOR Filed Nov. 21. 1960 r 5 Sheets-Sheet 5 55 mvuvrons.

HOWARD M. 8Ll$ skwnv J. HUBER BY FRZ'VK E. K OLINSKY ATTORNEY United States Patent Ofi ice 3,141,169 Patented July 14, 1964 3,141,169 OMNIDIRECTIONAL BEACON ANTENNA HAVING DIPOLE RADIATOR AND PARASITICALLY FED HORN RADIATOR Howard M. Bellis, Park Ridge, Erwin J. Huber, Midland Park, and Frank E. Kolinsky, Wanaque, N..I., assignors to International Telephone and Telegraph Corporation, Nutley, N.J., a corporation of Maryland Filed Nov. 21, 1960, Ser. No. 70,830 11 Claims. (Cl. 343-730) This invention relates to omnidirectional beacon antennas for use in producing a multiple modulation radiation pattern and more particularly, to antennas providing modulation over a relatively wide range of elevation angles in space.

Omnidirectional beacon systems, such as used in Tacan radio navigation systems, have provided a high order of directional accuracy. The directional nature of the antenna pattern is created by rotating some of the antenna elements about a central radiating element so that a modulation in space and in time is created on the antenna pattern. The degree of modulation in the antenna pattern is critical. Both the frequency of operation and the angle of elevation as measured from the antenna to an aircraft which is utilizing the beacon, have an effect on the quality of the modulation received by the aircraft. In the past, the use of antenna structures which were rather elaborate and rather large in size has provided satisfactory modulation performance. However, there are many applications where small size and weight requirements are imposed upon the beacon antenna. At the same time, the antenna must still provide proper and adequate modulation over a wide range of elevation angles and over two discrete frequency bands.

Therefore, a principal object of this invention is to provide an omnidirectional beacon antenna providing modulation which is neither too deep nor too shallow for operation over two discrete separated frequency bands.

It is another object of this invention to provide a novel central radiating structure which in cooperation with special antenna modulation elements provides various amounts of modulation at various different space elevation angles while at the same time confining the antenna structure to an appreciably smaller and more compact construction.

According to one aspect of the invention, an antenna system is provided utilizing a stationary dipole-like central radiator mounted above a radiating horn structure. The amount of radio frequency energy fed to the horn and transmitted by the horn is controlled by the coupling arrangement between the dipole and the horn. At the same time, the top surface of the horn is utilized as a counterpoise surface for shaping the radiation pattern of the dipole itself.

In another feature of the invention, parasitic modulation elements are rotated with respect to the central radiating element to produce appropriate modulation patterns. Both conductive materials and dielectric materials are used to form these modulation elements. The construction of these parasitic elements utilizes several appropriately spaced parasitic elements which are electromagnetically coupled to both each other and the central radiating element so that each group of parasitic elements cooperates with the dipole and horn to provide the correct amount of modulation at each elevation angle.

The above-mentioned and other objects and features of this invention and the manner of attaining them will become more apparent and the invention itself will best be understood by reference to the following description of the invention taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a diagram illustrating a typical frequency spectrum over which such an antenna operates;

FIG. 2 is a greatly simplified cross-sectional view of the non-modulating portion of the antenna;

FIG. 3 is a side elevational view illustrating in some detail a complete embodiment of the invention;

FIG. 4 is a more detailed plan view of the central portion of the antenna of FIG. 3;

FIG. 5 is a perspective view from the top of the antenna of FIG. 3 showing some details of construction of the modulating elements;

FIG. 6 is a series of side elevational views of various parasitic element combinations and graphs of the corresponding modulation effects produced by the respective parasitic configuration;

FIG. 7 is an enlarged detailed view showing the parasitic arrangement of FIG. 6D.

FIG. 8 is a top perspective view of an antenna similar to that shown in FIG. 3 with an alternate modulating structure; and

FIG. 9 is an enlarged view in perspective of the modulating arrangement of FIG. 8.

Refer now to FIG. 1. FIG. 1 shows two discrete frequency bands separated by a certain frequency spectrum. The lower frequency band is centered about a first fre quency F and the second, higher, frequency band is centered about a frequency F The frequency values shown are typical of the actual requirements of the Tacan navigation system. The center frequency spectrum is utilized for communicating other information required in the sys tern, such as range information, identification, heading, altitude, and so on, and the antenna structure of the present invention transmits and receives over the entire frequency spectrum, from the lower frequency and of the band centered about F to the upper frequency end of the band centered about F But the present invention is particularly concerned with providing time and space modulation over the two discrete separated frequency bands shown. As an aircraft approaches a rotating beacon antenna, the apparent amount of modulation received at the aircraft will vary with the elevation angle in space due to the fact that the antenna pattern appears to become fore-shortened at higher and higher elevation angles. To provide adequate modulation over the required elevation angle range, special radiating structures must be used.

Refer now to FIG. 2. In FIG. 2 there is shown an antenna generally designated as 1. At 2 is shown a coaxial line to introduce RF energy to the antenna radiating structure. Transmission line 2 has an inner conductor 3 and an outer conductor 4 insulated from each other by the construction of the transmission line. The inner conductor 3 of the transmission line 2 is connected directly to a first radiating member 5. The radiating member 5 is made of a conductive material and has the shape of a circular cup, or cylinder, as illustrated. Spaced from the upper radiating member 5 is a second radiating member 6 also made of conductive material. The first member 5 and the second member 6 are spaced from each by a hollow angular ring of insulating material 7. The outer conductor 4 of the transmission line 2 is connected directly to the second radiating member 6 as shown. Located coaxially with the transmission line 2 and near the bottom edge of radiating member 6 is a sheet of conductive material 8. The conductive sheet or plate 8 has a circular aperture 9 therein as shown. The aperture 9 has diameter D D should be approximately one half wavelength. Spaced a distance L along the axis of coaxial conductor 2 is a second conductive sheet or plate 10. The conductive plate 10 also has a circular aperture 11 which is coaxial with the transmission line 2. The aperture 11 is considerably smaller in diameter than the aperture 9.

The radiating members and 6 together form a dipoletype of radiator. The length L of the upper member 5 is one-quarter wavelength. The length of the lower member 6 is shown as L and length L is critical to the operation of the invention. If the length L were exactly onequarter wavelength, 5 and 6 would form a simple one-half wavelength dipole. The upper surface 12 of the conductive plate 8 acts as a reflecting ground plane or counterpoise for the dipole 5 and 6. The use of a counterpoise surface such as 12 provides a highly conductive ground plane for the dipole 5 and 6. Thus radiation from the dipole 5 and 6 that initially travels towards the surface 12 will strike the surface 12 and will be reflected in the upwards direction. This will produce a relative strengthening of the radiation pattern of the dipole and counterpoise at higher elevation angles because the counterpoise 12 reflects upwards or gives an upward tilt to the radiation pattern of the antenna. This reflection of more energy into higher elevation angles helps to make the invention suitable for providing the proper modulation at high space elevation angles as will be shown below.

In addition the conductive plate 8 serves a further function. The other side 13 of the conductive plate 8 forms one inner surface of a horn structure 14. The horn structure 14 is made up of the conductive plate 8 and the conductive plate together. The horn structure 14 directs a relatively well-defined beam out in the direction parallel to the two sides 8 and 10 of the horn. The use of a horn structure 14 allows a strong well-defined beam to be propagated along the direction of the horn 14 as shown essentially in the horizontal direction. The method of feeding electromagnetic energy to the horn 14 is also important. Energy passes through the transmission line 2 and to the radiating members 5 and 6. Part of the energy from the dipole structure 5 and 6 is radiated out into space and part of the energy is first reflected off the counterpoise surface 12 and then also is radiated out into space, but a definite fraction of the energy which enters the transmission line 2 passes from the dipole structure 5 and 6 down to the aperture 9 and out of the horn mouth 14.

If the length L of the radiating dipole 6 were exactly one-quarter wavelength, there would be no standing waves created on the surface of the outer conductor 4 of the transmission line 2. The area 15 of the outer conductor 4 which is surrounded by the radiating cup-shape member 6 would form an exact one-half wavelength trap if the length L were one-quarter wavelength. Electromagnetic waves coming down the outer side 16 would pass along 17, the inner edge of the members 6. Since the waves would pass one quarter of a wavelength on the way down along the edge 16 and go back one quarter of a wavelength on the inner edge 17, a full half wavelength would be traveled and the waves would be in phase when they reached their starting point and the disc-like portion 18 of the member 6. However, if the length L is made slightly more or slightly less than one quarter of a wavelength, the waves that propagate along the side 16 and then 17 will not cancel and as a result, standing waves will be created on the area 15 of the outer conductor 4 and this standing wave will proceed to travel down the conductor 4. This is shown by the dotted lines 19. The dotted lines 19 indicate the approximate configuration of the electric field along the outer conductor 4 within the horn when the length L is other than one quarter of a wavelength. Standing waves 19 form a virtual feed point and in this manner energy is fed from the dipole 5 and 6 through the aperture 9 along the outer conductor 4 and then is propagated out into space guided by the edges 13 and 10 of the horn structure 14.

It can be seen that this method of directing and shaping the radiation pattern of the antenna assures that there will be complete phase and time coherence between the three sources of radiation in the antenna. Thus, there is radiation from the dipole 5 and 6 directly; into space;

there is reflected radiation from the counterpoise surface 12 and there is radiation out through the horn 14 into space. But these three types of radiation are all in definite phase relationship with each other and the amount of power which goes through each of the three paths is fixed and determined once and for all by the physical design of the antenna. The horn structure 8 and 10 can be physically supported in a variety of ways. The physical supports are not shown in the view of FIG. 2. In general, dielectric material should be used to support the horn structure 14. Also the input impedance of the horn as seen by the transmission line 2 can be made capacitive or inductive by making the length L less than or more than one quarter of a wavelength respectively.

Refer now to FIG. 3. FIG. 3 is a preferred embodiment of the invention showing a complete antenna system suitable for use as an omnirange beacon in an air navigation system, such as the well known Tacan. The antenna of FIG. 3 utilizes the principles of energy distribution and pattern shaping that was explained in connection with FIG. 2. FIG. 3, however, has many additional novel features and improvements which will be seen from the discussion below. FIG. 3 shows an antenna generally designated as 20. The antenna is mounted on a base structure 21. The base structure 21 contains an electric drive motor system to cause rotation of the antenna, as will be explained. The motor system is not shown. A coaxial transmission line 22 with an inner conductor 23 and an outer conductor 24 is shown passing through a bushing 25 on the bottom 26 of the antenna base 21. Transmission line 22 can pass through the center of a hollow-shaft motor which might be used for driving the antenna inside of the base 21. The coaxial transmission line 22 passes through the base 21 out through the driving flange 27 and emerges within the antenna structure proper itself as shown. Transmission line 22 and the radiating structure which it supports is stationary at all times and does not revolve. The driving flange 27 provides mechanical coupling to rotate the revolving part of the antenna. The moving rotor of the hollow shaft motor is mechanically coupled or geared to the driving flange 27 and techniques for such mechanical revolving drives are well known in the art.

The upper part of the transmission line 22 supports the radiating dipole generally indicated as 28. The radiating dipole 28 includes an upper radiating member 29 made of conductive material and a lower radiating member 30 made of conductive material. The members 29 and 30 are circular, cup-shaped, closed cylinders, as indicated. The radiating members 29 and 30 are spaced apart from each other by a dielectric, annular insulating ring 31. The inner conductor 23 of the transmission line 22 is connected directly to the upper radiating member 29 and the outer conductor 24 of the transmission line 22 is connected directly to the radiating member 30, as shown. The dimension of the dipole member 29 along the axis of the transmission line is L, and is equal to one-quarter wavelength of the operating frequency of the antenna. The length of the dipole radiating member 30 along the axis of the transmission line 24 is L and this length is slightly less or slightly more than one-quarter of a wavelength. If L is made less than one-quarter of a wavelength, a relatively capacitive impedance is presented to the transmission line 22. If L is somewhat more than one-quarter of a wavelength, a relatively inductive impedance is presented to the transmission line 22. A typical value of L for a capacitive impedance might be .225 Wavelength, and a typical value of L for inductive impedance might be .275 wavelength, for example.

The central radiating elements of the antenna, which have been just described, are surrounded by a dielectric cylinder 32. The cylinder 32 is concentric with the axis of the stationary transmission line 22 and there is a space between the inner wall 33 of cylinder 32 and the radiating members 29 and 30. Located parallel to the axis of the transmission line 22 and attached to the inner wall 33 of the cylinder 32 is an array of dielectric parasitic modulating elements, 34 and 35. The parasites 34 and 35 are dielectric rods of diameter D A top view of only the central portions of the antenna in enlarged scale is shown in FIG. 4. The cylinder 32 has a radius R The two parasites 34 and 35 are located with a mechanical separation of C degrees of arc. As shown, the angle C is 90 degrees of mechanical arc. The fundamental parasitic modulation elements 34 and 35 are used to create the fundamental harmonic modulation of the space radiation pattern of the antenna 20 as will be explained below. The dielectric cylinder 32 is actually constructed in an upper portion in 32a and a lower portion 32b as shown. The upper portion 32a has a flange 36 and the lower portion 32b has a flange 37. These flanges are firmly attached to a conductive metal plate 38. The conductive plate 38 has an aperture 39 through which passes transmission line 22 as indicated. The flanges 36 and 37 are attached to the plate 38 near the inner edge of the aperture 39. The top surface 40 of the plate 38 forms a conductive counterpoise surface for the dipole 28. There is also present a second metal plate 41.. The conductive plates 38 and 41 thus form a horn structure. The plate 41 has a smaller aperture 42 through which passes the transmission line 22. A dielectric cylindrical cup 43 is formed of an outer dielectric cylinder wall 44 and a flat circular portion 45. The flat dielectric plate 45 of the cup 43 rests on top of the lower conductive plate 41. The plates 38 and 41 and 45 are rigidly spaced apart from each other by a dielectric support cylinder 46 which has an upper flange 47 and a lower flange 48. The lower flange 48 of the dielectric support cylinder 46 rests on and is firmly attached to the dielectric plate 45 of the dielectric cup 43. Likewise, the dielectric flange 47 is firmly attached to the conductive plate 38. The dielectric cylinder 44 is also attached to the conductive plate 38 by dielectric corner pieces indicated as 49. The parts may be mechanically fastened to each other by gluing, by heat bonding, or by using dielectric doweling, for example. The only precaution to be observed is that conductive fastening parts must be used with care because there might be a possibility that these conductive parts would change the radiation pattern of the antenna in an unexpected manner. The lower conductive plate 41 is firmly attached to the driving flange 27. As shown in FIG. 5, a number of conductive parasitic modulating elements such as 50 and 51 are attached to the dielectric cylinder 44 and the dielectric parasitic modulating elements 34 and 35 are attached to the cylinder 32 as previously mentioned.

FIG. 5 shows a top perspective view of the antenna of FIG. 3. In FIG. 5 it will be seen that a plurality of parasitic elements, such as 50 and 51 are arranged in systematic groupings. The functioning of the grouping of the parasitic elements 50 and 51, etc., will be explained in connection with FIG. 5. Also shown in FIG. 3 is a short metal cylinder or wall 52 parallel to the axis of the transmission line 22 which supports a circular metal strip or plate 53. There is an air gap or space 54 between the inner side of the metal strip 53 and the underside of the conductive plate 41. The members 52 and 53 with the air space 54 form a half wave shorted stub for the radiation. The stub formed by 52, 53 and 54 acts as a reflector or trap for any radiation which tends to propagate downward around the edge of plate 41. Hence stray radiation is trapped and prevented from reaching the antenna base 55, 21 etc. where it could cause undesired noise pickup. It can be seen in FIG. 3 that the horn structure composed of plates 38 and 41 and spacing cylinder 46 along with the inner cylinder 32 and the outer cylinder 44 form a rigid, sturdy cage which is mounted for rotation on the driving flange 27 Thus the stationary elements of the antenna are the transmission line 22 and the central members such as 29, 31, 30, and the revolving horn structure rotates about the stationary members.

6 Cylinders 32 and 44 and the plates 38 and 41 rotate as one complete unit.

As the antenna is rotated by the driving flange 27, the inner dielectric parasites 34 and 35 on the cylinder 32 create a fundamental modulation of the radiation of the central elements. At the same time, the plurality of conductive parasitic elements, such as 50 and 51, create a harmonic modulation of the radiation from the central elements. The frequency of the fundamental and harmonic modulation is directly a function of the speed of rotation of the antenna structure. If the flange 27, for example, is rotated at the rate of 15 cycles per second, that is, 900 revolutions per minute, the parasites 34 and 35 will create a 15 cycle-per-second fundamental modulation on the antenna radiation pattern. The parasitic elements 50 and 51, on the other hand, will create a harmonic modulation whose frequency is the speed of rotation of the antenna multiplied by the number of groups used on the antenna. If nine such groups of parasites are used, as indicated in FIG. 5, the harmonic modulation would be 135 c.p.s. It should be noted that the fundamental parasites 34 and 35 are not operating in the usual manner as the fundamental parasites of the prior art Tacan antennas. Since there are two fundamental parasites 34 and 35, it might be expected that they would create a 30 cycle-per-second modulation, that is, two times the speed of rotation, in other words, two times 15 cycles per second. This is not actually the case, however. In FIG. 4, the mechanical spacing of '90 degrees of are between the parasites 34 and 35 and the diameter D of each parasite have been chosen so that the two parasites 34 and 35 operate as one unit and not as two separate modulation elements. The degree spacing causes the 30 c.p.s. modulation from each (34 and 35) to cancel. The two parasitic rods 34 and 35 are electromagnetically coupled to each other over the two frequency bands of operation. Because of their diameter D; and their spacing angle C, the two parasites 34 and 35 actually appear to the central radiating elements, such as 29 and 30, as one broad fundamental parasitic element. Thus, the parasitic elements 34 and 35 act like a single broad parasite extending over the angle C and they create a first harmonic fundamental modulation rather than a second harmonic modulation. The angle C should preferably be 90 degrees. The use of the two parasitic elements 34 and 35 has a considerable advantage in that at the higher elevation angles, the lobe in the space radiation pattern due to the fundamental parasites is much broader in space in mechanical degrees. This leads to a higher and more acceptable percentage of modulation of the carrier frequencies at higher elevation angles with respect to the antenna. It will be remembered that the problem of producing a correct amount or depth of modulation over a wide range of space elevation angles was one of the defects of the prior art. The novel arrangement of the parasites 34 and 35 and their electromagnetic coupling to each other and the central radiating elements produces sufiiciently large depth of modulation at high space elevation angles for the fundamental frequencies of the modulation. The use of two separate parasitic elements 34 and 35 to create the fundamental modulation also has the advantage over using one large dielectric slug of a saving in weight. Since the fundamental modulation element represents essentially an unbalanced weight on the rotating antenna, it is desirable to keep any unbalanced weights as small as possible so as not to create disturbing torques on the antenna.

The antenna of FIG. 3 also has a shielding surface 55 made of conductive material which forms part of the casing for the motor compartment in the supporting structure for the antenna. The conductive shield 55 prevents interaction between the antenna and other electronic gear which might be found in the base case 21. It will be noted that the antenna of FIGS. 3 and 4 and 5 is an extremely small, compact and rugged structure. The height of the RF antenna assembly alone is only 12 inches as indicated, with a 30 inch diameter for the cylinder 44 and an overall height of 34 inches. To achieve such a compact physical design and at the same time provide an effective radiation pattern over the appropriate frequency band and over suitable elevation angie ranges has required the novel approach indicated.

The operation of the antenna of FIGS. 3 and 4 may be briefly summarized as follows. RF energy at the desired frequency is applied to the antenna by means of the transmission line 22. This energy passes up through the transmission line through case 21 and the driving mechanism and is supplied to the dipole 28 through elements 29 and 30. A portion of this energy is radiated directly into space. Another portion of this energy is radiated outward from the dipole elements 29 and 30 and it strikes the counterpoise surface 40. It is then reflected in an upward or tilted manner and recombined with the portion of directly radiated energy from the dipole 28. In addition because of the asymmetrical dimension L of the dipole element 30, a fraction of the RF energy supplied to the antenna is propagated by standing waves down the outside conductor 24 of the transmission line 22 and then outward through the horn 38 and 41. This portion of the energy is directed into a relatively well-defined beam progressing substantially parallel to the sides of the horn, i.e., in the horizontal direction. At the same time, all of the three types of radiation just described have a fundamental modulation imposed on them at the same time and in the same phase by the fundamental parasitic modulation elements 34 and 35 which rotate along with the cylinder 32, and hence create a fundamental modulation given by one times the speed of rotation of the antenna. For the case of an antenna rotated at 900 r.p.m., a cycle-per-second modulation is produced by the parasites 34 and 35. Note that all three of the types of radiation We have described are intercepted by the parasites 34 and and hence they have this modulation impressed on them immediately before they are directed through the three diverse paths of direct radiation, reflection off the counterpoise 40, and direction by the horn 38 and 41. Hence when these three radiation sources recombine at some distance from the antenna, the modulation imposed on them is assured of having the same and the proper phase. FIGS. 3, 4, and 5 show a preferred embodiment of our invention. However, the complete operation of the antenna of FIGS. 3, 4, and 5 will be best understood after the detailed discussion of the parasitic elements shown in FIG. 5.

FIG. 5 shows a group of parasitic elements generally designated as 56 mounted on the dielectric cylinder 44. Other identical groups of parasitic elements such as 57, 58, 59, 60, 61, 62, 63, 64 are equally distributed around the circumference of the cylinder 44. The groups 61, 62, 63 are on the far side of cylinder 44, and do not show in the view of FIG. 5. Groups 60 and 64 are shown partially. For use in the Tacan system, there are a total of nine such groups to produce ninth harmonic modulation, that is, nine times 15 cycles per second, or 135 cycle-persecond modulation, but any number of groups can be used as desired to produce other harmonics for a particular antenna. The parasites and 51 were shown in FIG. 3 in the side elevational view as members of one of the identical parasitic groups such as 56.

FIG. 7 shows one group from FIG. 5 isolated and enlarged for clarity. FIG. 6 shows in steps various parasitic arrangements and along side it are shown curves of frequency as abscissa versus antenna response at various elevation angles as the ordinate. FIG. 6 shows the actual development of the preferred parasitic grouping indicated in FIGS. 5 and 7. To understand the operation or the grouping of FIGS. 5 and 7, we now discuss FIG. 6 step by step to show the development of such a parasitic group. The two discrete frequency bands are indicated on the abscissa in FIG. 6. The ordinate indicates elevation angles in degrees. The various shaded areas indicate the type of response from the antenna. This is a response which would be experienced by a receiver located some distance from the antenna, for example, a receiver carried in an aircraft which was utilizing the antenna as a navigational aid as in the Tacan system. The series of curves in FIG. 6 are based on actual experimental data and also on theoretical calculations, and several actual models of the antenna were built and extensively tested and operated. A portion of the frequency scale has been omitted in the unused receiving band which separates the two frequency bands which are used. The code of FIG. 6 indicates that four types of effect can be achieved by the parasites, i.e., the harmonic cycle-per-second modulation may be usable, or one of three undesirable conditions may exist. There may be excessive modulation, the modulation may be out of phase, or there may be a frequency region in which the modulation is in process of undergoing a shift in phase. If the modulation is excessive, the receiver at an aircraft utilizing the beacon will experience difliculty in using or detecting the modulation signal from the carrier. 1f the modulation is not of the correct phase, this also causes difficulty in detection and introduces an error in the reading in the instruments of the aircraft. Only the usable areas of response are free from spurious defects.

Refer to FIG. 6a. In our discussion of parasitic modulation elements, it is well to define the terms director and reflector. A director is defined as follows: If when the antenna is transmitting a parasitic element is so placed that it will produce maximum radiation from its associated driver and if it operates to reinforce energy going from the driver element toward the parasitic element itself, then the parasitic element is referred to as a director. A reflector is defined as follows: If, when the antenna is transmitting a parasitic element is so placed that it will produce maximum radiation from itself back toward the driver, it is called a reflector. The reflector operates to decrease the energy going from the driver element toward the parasitic element itself. In the present case, the driver is the central elements, such as 29 and 30, and the parasitic elements 50 and 51, for example, might be either reflectors or directors depending upon their physical characteristics and their placement with respect to the central elements. FIG. 6A shows a group of parasitic elements 65 and 66. Parasitic elements 65 and 66 are conductive directors. Nine such sets or groups of conductive directors, such as 65 or 66, produce good modulation over the low frequency band, but at low space elevation angles at the higher frequency end of the high frequency band the modulation is out of phase as shown and is hence undesirable. Also, there is a region over some elevation angles in the high frequency band where the modulation occurs either at the incorrect phase or out of phase (i.e. a phase shift region). The arrow indicates the undesirable effects in the high frequency band. This condition cannot be easily corrected using only two parasites without producing excessive modulation on the low band. It is more desirable to have excessive modulation at the higher frequencies of the high frequency band where a parasite of very short length could be used to correct the situation by subtracting modulation from the high frequency band while having little effect on the low frequency band operation.

For this reason the arrangement shown in FIG. 6A was modified as shown in FIG. 6B. Here is shown two directors 67 and 68 centered about a reflector 69. All the parasites are again conductive, the directors 67 and 68 are tilted to reduce their coupling to the reflector 69. Resulting harmonic modulation is of the proper phase at all required points but it is excessive at most medium and high space elevation angles in the high frequency band. This is indicated with a closely shaded area in the high frequency spectrum. The problem then is to achieve a proper depth of modulation, that is, neither too much modulation nor too little modulation over the entire frequency range. The depth of modulation can be measured as M/ C where M is the amplitude of the harmonic modulation and C is the amplitude of the carrier wave which is used. It is highly desirable to maintain this ratio between 12 and 35 percent. Too little modulation prevents the receiver using the antenna from detecting the harmonic modulation signals and hence prevents a determination of bearing for navigation purposes. However, too much modulation prevents the reception of the carrier and prevents proper utilization of distance measuring and other circuits which must be used. Hence, the ratio, or depth of modulation, must maintain between certain limits. The arrangement of FIG. 6A can be used to vary the degree of modulation by changing the spacing between the elements or by changing the tilt angle. However, the coupling between the elements in each set of FIG. 6B was extremely critical and it was difiicult to actually accomplish such a correction. To correct this, the arrangement of FIG. 6C was used.

In FIG. 6C directors 70 and 71 are tilted and placed on either side of reflector 72. In addition, a shorter parasite 73 was placed immediately below the parasite 72. The parasite 73 adds greater depth of modulation at low space elevation angles in the high frequency band. Likewise, the parasitic element 73 subtracts modulation at all other angles. In the low frequency band, the element 73 operates to increase depth of modulation at high spaced elevation angles in the low frequency band improving the vertical coverage. This is shown by the fact that considerably higher vertical angles are covered by FIG. 60 as compared to FIGS. 6A and 6B. However, it will be seen that for certain frequencies in the high frequency band and at a certain of elevation angles, there is still excessive modulation as indicated by the arrow.

To further improve the performance, the parasitic group shown in FIG. 6D is used. Two conductive directors 74 and 75 are tilted and placed on either side of a reflector 50. A short element 51 is placed immediately underneath the reflector 50, to subtract modulation as explained in connection with FIG. 6C. In addition, conductive elements (76 on one side and 77 on the other side) are located 20 degrees of are away from the elements 50 and 51. The group of FIG. 6D thus has six parasitic elements. The identical parasitic elements 76 and 77 operate to subtract modulation at low and medium space elevation angles over the upper half of the high frequency band and also to lower the remaining high degree of modulation to usable levels. It can be seen in FIG. 6D that the resulting radiation pattern has no areas with undesirable characteristics and also the vertical coverage has been extended to relatively large vertical space angles of elevation. Elevation angles on the order of 55 degrees or more above the horizon can be successfully covered. The elements 50 and 51 were discussed and are shown in FIGS. 3 and 5. The entire arrangement is shown enlarged, located on the surface of the cylinder 44 in FIG. 7. It will be noted that there are actually nine elements, such as 77 for example and nine elements such as 76, in the antenna of FIGS. 3 and 5. In other words, the element 77 serves jointly the parasitic group both to the left of it and to the right of it. Likewise, the element 76 functions to serve both the parasitic group on its right and on its left in a balanced manner. This is also shown by FIG. 5.

It will also be noted that only the parasitic element 51 is actually located in front of the mouth of the horn structure 38 and 41. This is highly desirable because at low space elevation angles, that is, substantially in front of the horn 38 and 41, it is not ordinarily too difiicult to achieve the proper amount of modulation. This can be provided by the element 51. The conductive plate 38 to a certain extent prevents modulation elements, such as 76, 74, 50, 75 and 77, from having the undesirable elfect of producing excessive modulation at low space elevation angles. However, at higher space elevation angles, it is much more difiicult to achieve enough percentage of modulation. This phenomena is well known and is due to the apparent foreshortening of the radius of rotation of the antenna as it is viewed from over-head. For this reason, the elements above the counterpoised plate 38 receive radiation directly from the dipole unit 28 and also receive radiation which is reflected off the counterpoise surface 40 of the plate 38, strengthening the percentage of modulation experienced at relatively high elevation angles. FIG. 7 shows in detail a typical parasitic group of the referred embodiment of our invention. Nine such groups were used. The preferred dimensions are given in FIG. 7 and the angle of tilt E of the elements 74 and 75 is given. A typical value for the angle B would be 30 degrees. The parasites are made of a conductive material, such as brass in strips approximately .002 inch thick by %1" wide. Element 50 has a typical length of 3%". Thus the complete preferred embodiment of our invention is delineated in FIGS. 3, 4, 5 and 7 taken together. It should be noted the parasitic arrangement of FIGS. 5 and 7 shows the use of a fundamental parasitic arrangement, such as the elements 34 and 35, which are made of dielectric material to produce the first harmonic fundamental modulation and which provide a cancellation of the second harmonic as explained. In addition, the harmonic modulation elements, such as the groups 56, 57 through 64, etc., are made of conductive materials and have the characteristic configuration shown in FIG. 7. The prior art has never utilized both dielectric parasites for fundamental modulation and conductive parasites for harmonic modulation. Superior performance is obtained by the present antenna as illustrated by the diagrams of FIG. 6 and the discussion above.

Another variation of construction of the antenna is illustrated in FIGS. 8 and 9. FIG. 8 is a top perspective view of the antenna of FIG. 3 but with the important change that a different type of harmonic parasitic element is used. The central element 29 and the fundamental parasites 34 and 35 are as shown in connection with FIGS. 3 and 5. For the alternate embodiment of the antenna, the side elevational view of FIG. 3 is correct except that a block of dielectric material will appear in the side elevational view in place of the relatively thin conductive parasitic elements 50 and 51. Referring to FIG. 8, there is shown a central radiating element 29, and fundamental parasitic elements 34 and 35 mounted on the inner di electric cylinder 32. On the outer dielectric cylinder 44, however, are mounted a number of parasitic modulation groups. One such group is 78. To produce ninth harmonic modulation, nine identical groups were used and they are shown generally as groups 78, 79, 80, 81, 82, 83, 84, 85, 86. Groups 81, 82, 83, 84, 85, and 86 are shown partially because of the perspective. A discussion of one group, such as 78, will sufiice to explain the operation of all of the groups which are similar to each other. In FIGS. 8 and 9 is shown a dielectric block of material 87 (with an adjusted dielectric constant) securely fastened to the dielectric cylinder 44. Glue or heat bonding or any suitable method may be used with the exception previously pointed out that conductive fasteners must be used with judgment lest the radiation pattern of the antenna be disturbed. The dielectric block has the dimension W on the side attached to the cylinder 44 as shown and in a radial direction, the block 87 has a dimension D as shown in FIG. 9. Typical values for W and D would be: W equal to 1% inch, D equal to one inch, and the height of the block 87 shown as L6 might be 7% inches, the top of the dielectric block being flush with the top of the cylinder 44 and the bottom of the dielectric block resting on the top surface 40 of the conductive plate 38. Mounted in the center of the block 87 and inward radially toward the central radiating elements is a straight vertical parasite 88. The conductive parasite 88 is cemented or pressed into the block 87. The length of parasite 88 would be typically 5 /2 inches. The height of the dielectric cylinder 44 above the counterpoise 38 is shown as L6 and is approximately /2 wavelength. A value for L6 of 7% inches is typical. Mounted on the opposite, that is the outward side of the cylinder 44 are two conductive parasites 89 and 90, symmetrically mounted at an angle F with respect to each other in a V configuration. A typical value for the angle F between the elements 89 and 90 would be 60. The length of parasites 89 and 90 might be 6% inches typically. Thus, it can be seen a typical group of modulation elements, such as 78, comprises a relatively large dielectric mass 87 and three conductive parasites 88, 89 and 90, 89 and 90 being arranged in the symmetrical V configuration on the outer side of the dielectric cylinder 44. By utilizing both conductive and dielectric materials in one intimate grouping, a parasitic modulation group was provided which in effect provides stagger tuning of the modulation. The dielectric constant of the block 87 would typically have a value of 6.0. Blocks 87 with a number of dielectric constants ranging from 4.8 to 16 have been tested but a dielectric constant of 6 seemed to give the most efficient performance in achieving proper modulation over a wide space elevation angle and in antenna gain.

The cooperation of the elements of the parasitic group, such as 78, can be explained as follows. In the prior art, dielectric parasitic elements have been used principally to disturb the radiation pattern of the antenna. The use here is rather to smooth the radiation pattern of the antenna and to provide transition operation from one frequency to another by the use of the dielectric so that the three conductive parasites will operate as a properly functioning unit through the interaction with the dielectric material 87 but with reduced interaction with each other. A dielectric block, such as 87 alone, has been made to produce ninth harmonic modulation of constant phase over a frequency range slightly larger than that required for the Tacan operation. However, this did not produce a usable percentage of modulation while at the same time maintaining a constant phase harmonic modulation over the whole range of the Tacan spectrum. Previous theoretical work had indicated that an all band Tacan antenna could be built it a set of reflector parasites could be placed in the same nine locations as a set of director elements and if there was no serious interaction between the reflectors and the directors. Thus, the characteristics of the conductive parasites 88, 89 and 90 complement the characteristics of the dielectric parasite 87. The nine dielectric blocks, such as 87, provide excessive modulation at the high frequency band and provide insufficient degree of modulation which is also out of phase over the low frequency band. When a single vertical conductive parasite, such as 88, is centered on the inside surface of the block 87 and the V parasites, such as 89 and 90, are placed on the outside of the cylinder 44, partially in front of the dielectric block 87, proper depth of modulation is obtained over the entire Tacan frequency bands and over the required space elevation angles. The wires 89 and 90 and 88 alone tend to produce excessive modulation at the low frequency band and to produce insufficient degree of modulation which is out of phase at the high frequency band but the combination of the conductive parasites along with the dielectric block 87 as a dielectric parasite produces modulation that stays within the required ratio of from 12 to 35 percent degree of modulation over the required two-band Tacan frequency spectrum and at elevational angles well. in excess of 50 degrees space elevation. The outer wires 89 and 90 are tilted at the angle shown to reduce the interaction with the inner wire 88 and the dielectric block 87. Thus, the second complete embodiment of the invention is delineated by the side elevational view of FIG. 3 and FIGS. 8 and 9 taken together. As mentioned, the only change in FIG. 3 would be the showing of the dielectric block 87 at the cross sectional point where 51 and 50 now appear.

While we have described above the principles of our invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of our invention as set forth in the objects thereof and in the accompanying claims.

We claim:

1. An antenna system comprising first and second central radiating members emitting electromagnetic energy, said first member emitting energy into free space, a horn radiating structure, said first central radiating member being disposed apart from said horn radiating structure and separated therefrom by said second radiating member and means coupling a predetermined portion of said electromagnetic energy from said second central member to said horn structure for radiation therefrom.

2. An antenna system comprising a first central radiating member emitting electromagnetic energy, a second central radiating member in tandem with said first central radiating member emitting electromagnetic energy, a horn structure disposed adjacent said second radiating member, said first central radiating member being disposed apart from said horn radiating structure and separated therefrom by said second central radiating member, means coupling a predetermined amount of said electromagnetic energy from said second central member to said horn structure so that said coupled portion of electromagnetic energy is substantially directed into space by said horn structure.

3. An omnirange beacon antenna comprising first and second central elements emitting radiation, said first element emitting energy into free space, a counterpoise disposed to reflect a portion of said radiation from said second central element, a horn radiating structure, means coupling energy from said second central element to said horn structure whereby the radiation pattern of said antenna is created by the combination of the radiations from said first and second central elements and said counterpoise and said horn structure.

4. An antenna according to claim 3 further comprising a cylindrical member supported on said horn structure, means to rotate said horn structure and cylindrical member about said central element, a plurality of groups of parasitic modulation elements disposed on said cylindrical member, each said group of parasitic elements comprising a pair of linear conductive elements symmetrically disposed on opposite sides of a vertical line said line being located at a distance from said central element and lying parallel to said central element with the elements of said pair being closest together at their lowest point and slanting away from each other and said vertical line to form equal angles therewith, said group further comprising at least two linear conductive parasitic elements lying along said vertical line and between said pair of conductive elements, at least two linear conductive parasitic elements symmetrically disposed on opposite sides of said vertical line and said pair of linear conductive elements and parallel to said vertical line.

5. An antenna according to claim 3 further comprising a cylindrical member supported on said horn structure, means to rotate said horn structure and said cylindrical member about said central element, a plurality of groups of parasitic modulation elements disposed on said cylindrical member, each said group of parasitic elements comprising a pair of linear conductive parasitic elements symmetrically disposed on opposite sides of a vertical line, said line being located at a distance from said central element and lying parallel to said central element with the elements of said conductive pair being closest to- 13 gether at their lowest point and slanting away from each other and said vertical line to form equal angles therewith, a conductive parasitic element lying alongside of said vertical line and between said pair of conductive elements, a dielectric parasitic element lying along side of said vertical line and symmetrically disposed with respect to said vertical line and said pair of conductive elements.

6. An antenna system comprising a dipole, a transmission line coupled to said dipole, a counterpoise disposed below said dipole so as to direct a portion of the radiation from said dipole in a first direction, a horn structure having an aperture therein, means coupling said dipole to said aperture whereby a predetermined fraction of the electromagnetic energy from said dipole is coupled to said horn, a cylindrical member supported on said horn structure, a plurality of parasitic modulation elements disposed on said cylindrical member and means to rotate said horn structure about said dipole.

7. An omnirange beacon antenna system comprising a dipole, a transmission line coupled to said dipole, a counterpoise disposed so as to direct a portion of the radiation from said dipole in a first direction, a horn structure having an aperture therein and adapted to direct a predetermined fraction of the electromagnetic energy from said dipole in a second direction, means coupling said dipole to said aperture whereby said predetermined fraction of the electromagnetic energy from said dipole is coupled to said horn, a cylindrical member supported on said horn structure, means to rotate said horn structures and said cylindrical member about said dipole, a plurality of groups of parasitic modulation elements disposed on said cylindrical member, each said group of parasitic modulation elements comprising at least two linear conductive elements symmetrically disposed on opposite sides of a vertical line said line being located at a distance from said dipole and lying parallel to said transmission line, and at least two parasitic elements lying alongside of said vertical line and between said linear conductive elements.

8. An antenna according to claim 7 further comprising a radio frequency trap mounted on said rotating horn structure to prevent undesired radiation.

9. An antenna according to claim 7 wherein each of said parasitic elements lying alongside of said vertical line is a linear conductive parasitic element.

10. An antenna according to claim 7 wherein at least one of said parasitic elements lying alongside of said vertical line is a linear conductive parasitic element, and wherein at least one of said parasitic elements lying alongside of said vertical line is a dielectric parasitic element symmetrically disposed with respect to said vertical line and said pair of conductive elements.

11. An antenna according to claim 7 further comprising a pair of linear dielectric parasitic elements disposed parallel to said transmission line and nearer to said dipole than said plurality of groups of parasitic modulation elements, said pair of dielectric fundamental parasitic elements being disposed mechanical degrees of are from each other and so coupled to each other and said dipole that they produce a fundamental modulation of said radiation from said dipole.

References Cited in the file of this patent UNITED STATES PATENTS 2,866,194 Stavis et al Dec. 23, 1958 2,889,552 Thomas et al. June 2, 1959 2,935,745 Kelleher et a1. May 3, 1960 FOREIGN PATENTS 548,193 Great Britain Sept. 30, 1942 

1. AN ANTENNA SYSTEM COMPRISING FIRST AND SECOND CENTRAL RADIATING MEMBERS EMITTING ELECTROMAGNETIC ENERGY, SAID FIRST MEMBER EMITTING ENERGY INTO FREE SPACE, A HORN RADIATING STRUCTURE, SAID FIRST CENTRAL RADIATING MEMBER BEING DISPOSED APART FROM SAID HORN RADIATING STRUCTURE AND SEPARATED THEREFROM BY SAID SECOND RADIATING MEMBER AND MEANS COUPLING A PREDETERMINED PORTION OF SAID ELECTROMAGNETIC ENERGY FROM SAID SECOND CENTRAL MEMBER TO SAID HORN STRUCTURE FOR RADIATION THEREFROM. 